The invention relates to an electrical circuit containing temperature compensating resistor networks which stabilize the frequency of a signal generated by a crystal oscillator. Such a circuit is generally found in communications devices where frequency stabilization is essential. In particular, temperature compensating circuitry is found in communications devices that are used in a mobile environment and thus exposed to wide variations in environmental temperature. Generally, temperature compensating circuits consist of thermistors, resistors, and capacitors. Thermistors are semiconductors that vary in conductivity with changes in temperature. A thermistor has a negative coefficient of resistance.
Referring to FIG. 3, a conventional direct-type temperature compensating circuit includes a temperature compensating circuit P1 in series between ground and an AT-cut crystal oscillator P4. Temperature compensating circuit P1 includes a low-temperature compensating part P2 and a high-temperature compensating part P3 connected in series with each other. Low-temperature compensating part P2 includes a capacitor PC.sub.1 and a temperature compensating resistor network PRN.sub.1 connected in parallel. Temperature compensating resistor network PRN.sub.1 includes a thermistor PRT.sub.1 and a resistor PR.sub.1 connected in parallel. High-temperature compensating part P3 includes a capacitor PC.sub.2 and a temperature compensating resistor network PRN.sub.2 connected in parallel. Temperature compensating resistor network PRN.sub.2 includes a thermistor PRT.sub.2 and a resistor PR.sub.2 connected in series.
Temperature compensating circuit P1 controls the oscillation frequency of crystal oscillator P4 by capacitive loading of a crystal. The effect of the capacitance is controlled by a temperature responsive parallel resistance. As the ambient temperature decreases, the resistances of thermistors PRT.sub.1 and PRT.sub.2 increase. Thus, changes in the resistance of thermistor PRT.sub.1 compensates the oscillation frequency of crystal oscillator P4 at low temperatures. The relatively high resistance of the series combination of resistor PR.sub.2 and thermistor PRT.sub.2 makes high-temperature compensating part P3 relatively ineffective at low temperatures. At higher-than-ambient temperatures, the low resistance of thermistor PRT.sub.1 dominates the resistance of the parallel combination of thermistor PRT.sub.1 and resistor PR.sub.1, while the decreasing resistance of thermistor PRT.sub.2 compensates the oscillation frequency of crystal oscillator P4 at high temperatures.
Referring now to FIG. 4, a frequency-temperature profile depicts the normalized frequency deviation versus temperature for both compensated and uncompensated circuits. An uncompensated circuit produces a cubic polynomial curve as shown by curve A. A direct-type temperature compensating circuit produces a relatively flat curve as shown by curve B.
Low-temperature compensating part P2 functions at lower than ambient temperatures (25.degree. C.), and high-temperature compensating part P3 functions at greater than ambient temperatures, thereby maintaining the oscillation frequency at high and low temperatures close to the oscillation frequency at a reference ambient temperature.
The imaginary part of the impedance between terminals a and b or terminals c and d depends upon the capacitances of capacitors PC.sub.1 and PC.sub.2 in parallel with their respective resistor networks in low-temperature compensating part P2 and high-temperature compensating part P3. The effects of the capacitors between terminals a and b and between terminals c and d vary with the change of resistance in temperature compensating resistor networks PRN.sub.1 and PRN.sub.2. Therefore, the frequency-temperature profile depends on the capacitances of capacitors PC.sub.1 and PC.sub.2 and the characteristics of temperature compensating resistor networks PRN.sub.1 and PRN.sub.2.
Theoretically, a flat compensation profile can be obtained. In practice, however, it is difficult to determine the proper values for temperature compensating resistor networks PRN.sub.1 and PRN.sub.2 and capacitors PC.sub.1 and PC.sub.2. As a result, the values for resistors PR.sub.1 and PR.sub.2 are determined by repeated trial and error until the desired compensation profile is obtained.
In a typical measuring procedure, certain initial resistors are used as resistors PR.sub.1 and PR.sub.2 in a circuit. The temperature is varied systematically while an initial compensation profile is measured. New resistance values are calculated based upon the initial resistance values and profile results, and new resistors are installed in the circuit. These trial and error procedures are repeated until a proper compensation profile is obtained. Having to determine the resistance values by this method decreases productivity and increases the production cost of the final product.
Another procedure uses chip resistors whose resistances are changed with a laser beam. Instead of replacing resistors in the testing circuit, the resistance of the chip resistor is adjusted by laser cut to obtain the proper compensation profile. However, although the resistance of resistor PR.sub.2 is easily measured, it is impossible to measure the resistance of resistor PR.sub.1 since it is in parallel with thermistor PRT.sub.1. The resistance value cannot therefore be easily duplicated in a similar circuit.